It has long been recognized that there is a normal transcutaneous electric potential associated with mammalian skin (see, e.g., Robert Edelberg, in, The Biophysical Properties of the Skin, Harry Elden (ed.), Chapter 15, Wiley Interscience, 1971). This potential is to a great extent influenced by the presence of sweat glands and hair, and thus the strength of the potential may differ both spatially and temporally on the skin. However, even in nonglandular areas of the skin, there is a fairly strong, measurable current produced across the epidermis, in essence forming an intrinsic skin battery. Although most measurements have been conducted on non-human mammals, considerable evidence indicates that the same type of battery exists on human skin as well (Barker et al., Am. J. Physiol. 242: R358-R366, 1982). Such batteries have been known to exist in amphibians, where they apparently serve a function in sodium uptake and appendage regeneration. However, their purpose in a nonaquatic vertebrate was not readily apparent. Based on observations of fairly strong voltage gradients at the margins of wounds, Barker et al. and others have suggested that in mammals the skin currents may be important in the process of wound healing.
Further evidence of the importance of electrical currents in the maintenance of healthy skin has been shown in the successful use of electrotherapy in treatment of skin damages. For example, Carley and Wainapel (Arch. Phys. Med. Rehabil. 66: 443-446, 1985) have shown that treatment of indolent ulcers with low intensity direct current significantly increased the healing rate of those treated individuals relative to individuals treated with conventional therapy, with a concomitant reduction in pain and discomfort in those treated with electrotherapy. Similarly, Grace Chao et al. (Connective Tissue Research, 48: 188, 2007) noted the effects of applied DC electric field on the ligament fibroblast migration and wound healing processes, and Alvarez O M et al. (J. Invest. Dermatol., 81(2): 144-148, 1983, August) demonstrated that the healing of superficial skin wounds was stimulated by external electrical current.
Therefore, it appears that the maintenance of an electric current on the skin is associated with the continued well-being of undamaged skin, and that application of an electrical current to damaged skin can be highly beneficial to the healing process of such damaged skin. In addition to the reported treatment of ulcers, there are a number of other skin conditions involving irritation or inflammation which could also potentially benefit from preventive and/or therapeutic application of a low intensity current. However, the means for delivery of electrical current to skin reported in the medical literature typically involve the use of external power source and monitoring devices, which would be prohibitively expensive and complicated for the treatment of less serious skin damages, which are not life-threatening but nonetheless painful and irritating.
Although dermal patches featuring ultra-thin power supplies and electrodes printed or laminated onto elastic and flexible plastic substrates have become commercially available in recent years, such dermal patches are mostly designed for aiding transdermal delivery of active cosmetic or pharmaceutical ingredients into the skin. FIG. 1 illustrates a typical dermal patch used for conventional transdermal delivery of active cosmetic or pharmaceutical ingredients into the skin, which contains a printed, ultra-thin micro battery connected to an anode and a cathode that are in direct contact with the skin surface. The electrical potential differences between the anode and the cathode, as provided by the micro battery, generate an electric current that flows from the anode through the skin to the cathode, which in turn asserts repulsive electromotive forces on charged active cosmetic or pharmaceutical ingredients on the skin surface, i.e., the positively charged anode will repel positively charged active cosmetic or pharmaceutical ingredients into the skin, while the negatively charged cathode will repel negatively charged active cosmetic or pharmaceutical ingredients into the skin.
However, the current intensity achieved by such conventional dermal patch is typically in the milli-ampere (mA) range, which may cause irritation or inflammation of the skin. Further, because the electric current generated by such conventional dermal patch flows through the skin, the current intensity is significantly affected by various factors, such as the pH, moisture content, and resistance of skin, which can vary widely from person to person and even for the same person at different times of the day. In addition, the conventional dermal patches still require presence of power supplies, which significantly increase the complexity and costs of manufacturing and present additional modes of failure.
In contrast with the conventional devices described hereinabove, the present invention provides a simple and novel system for consistently generating a mild electrical current that flows across the skin surface, instead of through the skin, in a sustainable manner without the need for any external power source. Inventors of the present invention have discovered that such mild cross-flow electrical current is surprising and unexpectedly effective for preventing or treating skin damage and improving skin quality, even in absence of any cosmetic or skin care actives.